Plot hole: How does the little girl Nina know Ferdinand's name if only the animals can communicate with each other? They're all calling him Ferdinand before she "names" him that.
Continuity mistake: When they jump in the pool, they drop the guitar. When they jump out their guitar is no longer in the pool.
Other mistake: When Captain Underpants falls in the Turbo Toilet 2000's mouth which has a liquid in it, he doesn't turn into Mr. Krupp.
Continuity mistake: When Mariah is talking to the girls on the bridge at the beginning of the movie, the girl in green is holding her leash (her dog's that is) with her right hand in the shots from behind, the left hand in others including the wider ones from the protagonist's height. (00:02:25)
Continuity mistake: Back at the bar, the table to the right of Ray and John where Ray's future boyfriend, Jacob, is sitting has a light sitting on it. The light is right in the center of the table. Then after John points to them guy sat that table and it shows them from a different angle, the light is suddenly closer to the edge and towards the right. (00:25:30)